Over the years, gas turbine engine manufactures have increased the temperature and pressure at which gas turbine engines operate to meet demands for more powerful and efficient engines. The increased temperatures and pressures have imposed rigorous operating conditions on certain engine parts, particularly turbine vanes and blades immediately downstream of a combustor. In modern engines, turbine vanes and blades may be exposed to temperatures near or above the melting point of the alloy from which they are made.
While manufacturers have been designing gas turbine engines that operate under very demanding conditions, they have been striving to improve gas turbine engine reliability and to extend maintenance intervals to improve the economics of operating gas turbine engines. Manufacturers have addressed both objectives by applying protective coatings to certain parts, particularly turbine vanes and blades. Initially, the coatings focused on providing oxidation and corrosion protection. Examples of these include diffusion aluminide coatings, MCrAlY coatings, where M is Ni, Co, Fe, or Ni/Co, and other metallic coatings. Commonly assigned U.S. Pat. Nos. 4,585,481 and Re 32,121, both to Gupta et al., describe such coatings. More recently, multi-layer, thermal barrier coatings that comprise an oxidation and corrosion resistant metallic bond coat and a ceramic top coat have been used. Such coatings are described in commonly assigned U.S. Pat. Nos. 4,321,310 to Ulion et al., 4,321,311 to Strangman, 4,401,697 to Strangman, 4,405,659 to Strangman, 4,405,660 to Ulion et al., 4,414,249 to Ulion et al., and 5,262,245 to Ulion et al. Thermal barrier coatings provide thermal resistance to the high temperatures in a gas turbine engine in addition to providing oxidation and corrosion resistance.
As with other gas turbine engine parts, gas turbine engine operators find it desirable to repair thermal barrier coated parts periodically to restore them to desirable conditions. An important step in repairing thermal barrier coated parts is stripping the thermal barrier coating. Both the ceramic and metallic portions of the coating are removed. The process for stripping the ceramic portion of the coating may include soaking the part in a solution of KOH for about 4 hours to about 8 hours at a pressure of about 375 psia to about 425 psia and a temperature of about 400.degree. F. to about 450.degree. F. The metallic portion of the coating may be stripped by soaking the part in a HCl solution for about 2 hours at atmospheric pressure and a temperature of about 140.degree. F. to about 160.degree. F.
Although the stripping process effectively removes the ceramic and metallic portions of the coating, it also removes a portion of the base metal under the metallic portion, thinning the exterior wall of the component. The thinner wall is acceptable as long as it meets applicable inspection criteria. To ensure that the component wall does not become too thin, a wall thickness inspection is used to determine the wall thickness remaining after stripping. Once the wall thickness becomes too thin, the part is scrapped. As a result, stripping and related blending operations limit the number of times a particular part can be repaired. Once the repair limit has been reached for a particular part, the part must be replaced with a new part, rather than being repaired.
If the part is repairable, it is routed through the rest of a repair cycle. The repair cycle may include operations such as weld repairs to fill cracks and/or restore tip dimensions, braze repairs to fill cracks and/or restore or change a vane's "class" ("class" is a measure of the amount of open area available for gas flow in an assembled gas turbine engine), tip repairs to restore abrasive tips, and other steps. Many of these operations are described in more detail in expired U.S. Pat. Nos. 4,008,844 to Duvall et al., 4,073,639 to Duvall et al., and 4,078,922 to Magyar et al. Naturally, the repair cycle also includes reapplication of both layers of the thermal barrier coating. Following repair, the parts are inspected again to determine if they are acceptable to return to service.
Although this type of repair cycle has been used successfully in many instances, it has several drawbacks. First, the use of chemical stripping can limit the number of times a particular part can be repaired. As a result, the operator may need to purchase a new part when the repair limit is reached. Second, removing and reapplying the metallic bond coat portion of the thermal barrier coating can result in cooling hole plugging (sometimes knows as "coat down"). Parts with plugged cooling holes are typically unacceptable for return to service as is and must either be reworked or discarded. Third, stripping and reapplying the coating can be expensive and time consuming. In this age of rapid turn times and "just in time" planning, the time it takes to perform a repair cycle that includes stripping and reapplication of the coating can be a major issue with some customers. In addition, coating application can generate waste material or by-products that require costly disposal.
Therefore, what is needed in the industry is a repair for gas turbine engine parts coated with a ceramic thermal barrier coating that increases the number of times a part can be repaired and is less expensive and time consuming prior art repair methods. Moreover, the repair method should not lead to the coat down phenomenon noted with prior art repair methods.